1. Field of the Invention
The present invention relates to a vector network analyzer (VNA) with components utilized to limit size, weight and power consumption. More particularly, the present invention relates to components utilized to provide such a VNA with two measurement ports and to enable the VNA to alternatively function to monitor the frequency and power level of a received signal.
2. Description of the Related Art
Wireless networks for telephone or computer local area network (LAN) communications have been adopted worldwide in the 0-3 GHz frequency range. For instance, cellular telephones in the United States operate in the 800 MHz range, pagers operate in the 900 MHz range, and personal communication services (PCS) telephones are allocated operation frequencies in the 1850-2200 MHz range.
Wireless communications networks such as for cellular telephones, pagers, or PCS require remotely located antennas which transmit signals from cell sites to provide services to individual users. To reduce service costs for the remote antennas, it is desirable to test the remote antenna and its associated microwave components at its remote location and locate and repair portions which fail.
VNAs enable a user to easily identify a fault and to measure the distance from the test device to a fault or discontinuity. A fault may result from environmental conditions such as corrosion of a connector, or from faulty installation or repair, for example, where a coaxial cable is punctured by an installer. Test measurements are first typically made using a VNA operating in the frequency domain to determine whether a fault exists as indicated by an undesirable standing wave ratio (SWR). If a fault is discovered, an analysis of the results derived from the frequency domain measurement is made in the time domain to locate the position of the fault.
A VNA also enables calibration to extend the test port connection to the end of a cable connected to the test port. By using a VNA to extend the test port to the end of the cable, errors in the cable will not be taken into account when measuring a device through the cable. Scalar devices which measure only amplitude do not enable extension of the test port to the end of a cable.
Because a remote antenna may be constantly providing signals for communications devices once it is installed, it is desirable that the remote antenna not be disabled for testing. By measuring both phase and amplitude, a VNA provides the ability to distinguish extraneous signals, enabling tests to be performed with the remote antenna active. Scalar devices cannot distinguish the extraneous signals, requiring that scalar tests be performed with the remote antenna disabled.
Most VNAs are large and not easily transportable. Typical VNAs have a housing greater than two feet on a side, and with a power supply may weigh 50 pounds or more. The VNAs are typically transported by truck to the remote antenna sight and carried by two people to the remote antenna. With remote antennas located on top of towers, transportation of the large VNA proves especially difficult.
The SiteMaster manufactured by Wiltron Company of Morgan Hill, Calif., in contrast, is a single measurement port VNA having dimensions equal 10 or less than 8 inches by 6 inches by 21/2 inches and a weight of less than three pounds, including batteries, when operating over a 25 MHz to 3.3 GHz frequency range. With such weight and size, the SiteMaster can be easily transported by one person to make test measurements at a remote sight. Components used in the currently manufactured SiteMaster are described in the cross-referenced handheld VNA application.
With one VNA measurement port, the SiteMaster can be utilized to discover the existence of a fault or discontinuity using frequency domain measurements, and locate the position of the fault using time domain measurements. The SiteMaster can also be used to extend a test port to the end of a cable, and to make measurements on an active device by distinguishing extraneous signals.
However, it would further be desirable to have a handheld VNA with two measurement ports enabling transmission measurements to be made. For instance in a cell site, a low noise amplifier (LNA) is utilized to provide a signal from a transmitter to an antenna. Although no faults may be discovered when making a one port reflection measurement through the LNA, the LNA may not be properly amplifying a signal from the transmitter, a fact which cannot be easily discovered using a one port VNA. By transmitting a signal through the LNA from a first VNA measurement port, and receiving the transmitted signal from the LNA at a second VNA measurement port, one can determine whether the LNA is properly amplifying an input signal.
A two measurement port handheld VNA would also be desirable to measure antenna gain at a cell cite. As with the LNA, when making a one port reflection measurement through an antenna, no faults may be discovered, but the power delivered by the antenna may be below a desired level in a particular direction from the antenna. By transmitting a signal through the cell site antenna from a first VNA measurement port, and receiving the transmitted signal with another antenna at a second VNA measurement port, antenna gain in different directions from the antenna can be determined.
Even if a cell site is functioning properly, signals from another antenna may be interfering with signals transmitted from or received by the cell site. A cell site antenna is typically located in close proximity with a significant number of other antennas. For instance, a remote location at which a cellular telephone cell antenna is located may also be the location for transmitter antennas for pagers, PCS, and AM and FM radio stations. Measurements at a cell site are, thus, also typically made to determine if interfering signals are present utilizing a spectrum analyzer.
The SiteMaster currently manufactured by Wiltron is designed to reject signals from external sources when making measurements, so the currently manufactured SiteMaster cannot be used to measure the frequency and power level of signals from external devices. Further, a spectrum analyzer typically uses upconverting and downconverting signals to reject undesired images generated when mixing. Such upconverting and downconverting typically requires generating a signal with a frequency significantly higher than a desired test signal. Components to generate such a high frequency signal if combined with components of a handheld VNA would add an unacceptable amount of weight and size to the handheld VNA.